Superabsorbent refers to a water-swellable, water-insoluble, organic or inorganic material capable of absorbing at least about 10 times its weight and up to about 30 times its weight in an aqueous solution containing 0.9 weight percent sodium chloride solution in water. A superabsorbent polymer is a crosslinked polymer which is capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure in accordance with the general definition of superabsorbent.
The superabsorbent polymers that are currently commercially available are crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution. As a result of these characteristic properties, these polymers are chiefly used for incorporation into sanitary articles, such as babies' diapers, incontinence products or sanitary towels.
For fit, comfort and aesthetic reasons and from environmental aspects, there is an increasing trend to make sanitary articles smaller and thinner. This is being accomplished by reducing the content of the high volume fluff fiber of these articles. To ensure a constant total retention capacity of body fluids in the sanitary articles, more superabsorbent polymer content is being used in these sanitary articles. As a result of this, superabsorbent polymers must have increased permeability characteristics while retaining other characteristics such as adequate absorption and retention.
Permeability is a measure of the effective connectedness of a porous structure, be it a mat of fiber of a slab of foam or, in this case, crosslinked polymers and may be specified in terms of the void fraction and extent of connectedness of the superabsorbent polymer. Gel permeability is a property of the mass of particles as a whole and is related to particle size distribution, particle shape, the connectedness of the open pores, shear modulus and surface modification of the swollen gel. In practical terms, the permeability of the superabsorbent polymer is a measure of how rapidly liquid flows through the mass of swollen particles. Low permeability indicates that liquid cannot flow readily through the superabsorbent polymer, which is generally referred to gel blocking, and that any forced flow of liquid (such as a second application of urine during use of the diaper) must take an alternate path (e.g., diaper leakage).
In particular, gel blocking is a well-known problem that may be associated with the use of superabsorbent polymers in absorbent articles such as diapers. Gel blocking occurs when rapid expansion of the superabsorbent polymer particles around the point of entry of body fluid into an absorbent article causes a closing of the interstitial spaces and pores in the SAP-fluff matrix. Since the transport of liquid by diffusion through swollen hydrogel is much slower than transport through the interstitial spaces, a sealing effect occurs in the area of fluid entry. This effect is referred to as gel blocking.
Transportation of liquid through swollen superabsorbent polymer particles themselves follows the laws of diffusion and is a very slow process which plays no role in the distribution of the liquid in the use situation of the sanitary article. In superabsorbent polymers, which cannot maintain an open bed structure to effect capillary transportation because of a lack of gel stability, the separation of the particles from one another has been ensured by embedding the superabsorbent polymer into a fiber matrix.
In diaper constructions, for what is called the next generation, there is less fiber material, or potentially none at all, in the absorber layer to assist in transportation of the liquid or maintenance of an open, fluid permeable structure. The superabsorbent polymer of these next generation diaper constructions must have a sufficiently high stability in the swollen state, generally called gel strength, so the swollen gel has a sufficient amount of capillary spaces through which liquid can be transported.
To obtain a superabsorbent polymer with high gel strength, the degree of crosslinking of the polymer may be increased, which necessarily results in a reduction in the swellability and the retention capacity. To achieve the increased permeabilities needed in extremely thin, next generation articles with low fiber content, current art has taught to increase the amount of crosslinking to have higher gel strength achieved, typically having a shear modulus of greater than 9,500 dynes/cm2. However the absorption and retention values of the superabsorbent polymers are reduced to undesirably low levels. It is an important goal of the art of making superabsorbent polymers to develop a polymer having a high absorption and retention capacity for liquid in the after-surface crosslinking stage and increased permeability properties. It has been found that by using new surface modifications to the SAP particles, results of higher permeabilities without very high gel strengths and the undesirable associated low absorption values are achieved.
It is therefore an object of the present invention to provide an absorbing polymer composition that exhibits excellent properties such as capabilities of maintaining high liquid permeability and liquid retention even when the superabsorbent polymer is increased in percent by weight based on the absorbent structure.